1. Field of the Invention
The present invention relates to information transmission in a cellular communication system of OFDM, especially to the transmission of paging information in a standby state.
2. Description of the Related Art
In orthogonal frequency-division multiplexing systems, all the information to be transmitted is transformed by the OFDM modulation unit with IFFT (Inverse Fast Fourier Transform) algorithm at the transmit apparatus. Then the modulated information is emitted out via radio transmit front-end before it reaches the radio receive front-end through wireless channel. The information finally is transferred by radio receive front-end to the OFDM demodulating unit for FFT (Fast Fourier Transform) operation. Such IFFT and FFT operations require complex calculation. The amount of calculation is large and hence the power consumption is high.
Furthermore, for purpose of high-speed data transmission, usually a big operation length is set for FFT operation, namely the number of subcarriers is rather big, such as 1024, 2048 and so on. Therefore the operation is more complex. If the time for an OFDM symbol is 0.1 second in a system, it will take 51200 times of complex multiplication and 102400 times of complex addition per second to accomplish a 1024-point FFT operation. Such complex operation will lead to great power consumption.
Especially in receive device, for instance a mobile phone, only demodulation is needed in sleeping mode (namely standby state) to get paging information. But if we conduct FFT operation every time when paging occurs, there will be great power consumption in the mobile phone's battery. Hence it will reduce the usable time of the battery after charge.